Mount Sizer is the highest point in Henry Coe State Park, a relatively uninteresting bump on the top of Blue Ridge. One can reach Blue Ridge from several directions, the shortest of which is to take Hobbs Rd from the Park Headquarters.
While the summit of Mt Sizer itself is nothing to write home about, the views along the ridge into the deeper Coe backcountry are amazing, and the recent fires have cleared the view even more.
This peak is best done in the fall/winter/spring time frame due to the summer heat, but it is accessible year round. Creek crossings can be high in the winter but offer places to refill the water bottles. Wildflowers are abundant in the spring.
From highway 101, take the East Dunne Avenue exit and head east into the residential area. Continue into the hills. You will see a sign pointing to Henry Coe at a 'Y' intersection. From here it is 10 twisty miles to the Park Headquarters.
There are several options for reaching the summit of Sizer. The shortest approach is to take Hobbs Rd from Headquarters - this will be an approximately 14 mile round trip hike with ~4500 ft of gain. Making a loop through Poverty Flat or China Hole will add a few miles but also offer some variety.
The Hobbs Rd Shortcut, which goes from Coyote Creek up to Blue Ridge, is often cited as the steepest trail in the Bay Area. It climbs 1700 ft in 1.3 miles, so if you want to make a loop choose your direction by deciding whether you'd rather climb or descend this grade.
This is a Poison Oak Sufferer friendly peak. You can reach it entirely by hiking on wide fireroad, and no bushwhacking through overgrowth is required (unless, of course, you choose to).
At the time of this writing, there is an $8 day fee for parking at Park HQ. No separate day hiking permit is required, but there may be requirements for an overnight stay.
There is a car campground at Park Headquarters, and numerous backcountry sites along the trails that approach Sizer. See the Coe Park website below for current rules and regulations regarding backcountry camps.